Low-alloy high-strength steel



United States Patent 3,379,582 LOW-ALLOY HiGH-STRENGTH STEEL Harry J. Dickinson, 1116 S. 41st St., Birmingham, Ala. 35222 No Drawing. Continuation-impart of application Ser. No. 402,613, Oct. 8, 1964. This application Feb. 15, 1967, Ser. No. 616,180

3 Claims. (Cl. 148-36) ABSTRACT OF THE DISCLOSURE A low-alloy, high-strength steel cooled from an austenitizing temperature and having a martensitic microstructure, and comprising from .20 to 30% carbon, .80 to 1.2% manganese, 3.25 to 4.00% nickel, 1.25 to 2.00% chromium, .25 to .50% molybdenum, .20 to 50% silicon and balance iron and residual amounts of other elements.

This application is a continuation-in-part of my prior application Ser. No. 402,613, filed Oct. 8, 1964, now abandoned and entitled, Low-alloy High-Strength Steel, and application Ser. No. 560,812, filed June 27, 1966, now abandoned and entitled, Low-allow High- Strength Steel and Process of Producing Same.

Background of the invention As is well known in the art, the selection of a proper balance of alloying elements to obtain uniform strength and toughness properties over a broad range of section thicknesses is very difficult. Structural metals used for critical high-stress applications, or in any application where overall reduced weight of the structure is desired,

must have extremely high strength and toughness proper- Brief summary of invention This invention relates to a low-alloy, high-strength steel having imrpoved strength, toughness and ductility as well as deep hardenability over a broad range of section thicknesses which is obtained by air-cooling, thus eliminating the expensive and troublesome task of quenching that can result in minute cracks, mixed microstructures, and high stress. I provide a novel, useful and economical combination of alloy elements which produces uniform hardness from surface to core with less brittleness at higher hardness levels and without the addition of elements, such as boron to obtain deep hardenability. Also, I provide a low-alloy, high-strength steel which has a micro-structure that enables machining at high hardness and which is weldable, including heavy sections, without preheating or post heating.

The steels produced in accordance with the present invention produce a yield strength of above 140,000 p.s.i. and a tensile strength in excess of 220,000 p.s.i. These improved strength properties, as well as improved toughness properties, are obtained by air-cooling from an austenitizing temperature, thereby eliminating the necessity of quenching and other complicated treatments. A further advantage of air-cooling to attain very high strength properties is that residual stresses are low relative to quenched steels. Furthermore, air cooling has the advantage of being less likely to cause distortion than liquid quenching, which is usually necessary to obtain effective hardening in commercial steels.

My improved strength and toughness properties result from the proportions of carbon, manganese, nickel, chromium, molybdenum and silicon, as set forth in the following compositional range:

TABLE I Range, percent Element: by weight Carbon 0.20 to 0.30 Manganese 0.80 to 1.20 Nickel 3.25 to 4.00 Chromium 1.25 to 2.00 Molybdenum 0.25 to 0.50 Silicon 0.20 to 0.50 Sulphur maximum 0.04 Phosphorus do 0.04

Balance iron and risidual amounts of other elements.

For the optimum combination of strength, toughness and weldability properties the preferred range percent by weight is as follows:

Carbon 0.24 to 0.26 Manganese 0.90 to 1.00 Nickel 3.40 to 3.60 Chromium 1.45 to 1.75 Molybdenum 0.25 to 0.35 Silicon 0.25 to 0.40

Strength, toughness and hardness propertie of my lowalloy, high-strength steel are set forth hereinafter in specific examples wherein the analysis of the alloy is as follows:

TABLE II Element: Percent by weight Carbon 0.24 Manganese 0.90 Nickel 3.45 Chromium 1.45 Molybdenum 0.30 Silicon 0.28

The alloy composition set forth in Table II was normalized by heating to 1750 F. This temperature was held for one hour and the composition was then air-cooled.

Tensile and toughness specimens were machined from one-quarter inch plates which were rolled from the steel thus produced from the composition set forth in Table II. The specimens were longitudinal; that is, the long dimension was parallel to the plate-rolling direction. Room temperature tensile properties were determined in a 120,- 000 pound capacity Universal testing machine at a nominal strain rate of 0.01 in./in./min. The tensile-property data are shown in the following table: TABLEIII.ROOMFTEMPERATURE TENSILE PROPERTIE 5 OF NORMALIZED ALLOY STEEL 0.2% Offset Yield Ultimate Tensile Elongation in 2 in.,

Strength, p.s.i. Strength p.s.i. Percent By tempering the normalized steel for 2 hours at 500 F., the yield strength can be raised to 170,000 p.s.i. or higher, with only a slight decrease in ultimate tensile strength (to about 215,000 p.s.i.). The elongation is essentially unaffected by the tempering treatment.

In actual practice, I find that alloy-steel made in accordance with my invention has good impact properties. That is, at temperatures ranging down to F., the

impact strength is approximately ft.-pounds or higher. Also, at room temperature the percent of fibrous fracture is 100%. My improved alloy steel has extremely good fracture-toughness properties, even down to low temperatures. The fracture toughness, Which is a measure of a materials resistance to unstable crack propagation, was determined with pre-cracked charpy bars and found to be 600 in.-lb./in. or higher at temperatures of -100 F. and higher.

It has been determined by magnetic-permeability measurements that normalizing my steel from 1750 F. results in a completely alpha-iron structure. In other words, no austenite is retained on air-cooling to room temperature. The microstructure of my steel is essentially a martensite of relatively low carbon content, and it is this structure that is responsible for the excellent combination of strength and toughness developed by normalizing.

As a further example of the improved hardness properties of my alloy-steel, a three-inch diameter bar formed from the composition set forth in Table II was normalized at 1750 F. for one hour and air-cooled. The sample was sliced midway the length thereof and a hardness traverse was run across the bar diameter on the cut surface. The hardness ranged from Rockwell C 40 to 42. The hardness in the center of the bar was C 40. Midway between the center of the bar and the surface thereof, the hardness was C 4042. Near the surface of the bar (within inch) the hardness Was C 40. Similar tests were made with samples of five-inch and eight-inch diameter bars, normalized from 1750 F. It was found that the lowest hardness developed in the five-inch and eight-inch sections was at least C 38, which was measured at the bar center. It will thus be seen that excellent deep hardening is obtained on normalizing my steel in bar sizes up to eight inches in diameter or larger.

Specimens of the steel made from the composition set forth in Table II were rolled at various temperatures after first being preheated to a temperature of 1950 F. and the following results were obtained after air cooling from the rolling temperature indicated:

TABLE IV Rolling temperature, F.: Rockwell C hardness TABLE v Element: Percent by weight Carbon 0.30 Manganese 0.93 Nickel 3.30 Chromium 1.35 Molybdenum 0.26 Silicon 0.36

Test samples were taken from the center of a 1 inch plate of the alloy composition set forth in Table V and normalized one hour at 1700 F., air cooled and then tempered at 450 F. for two hours to a high hardness (Rockwell C 48).

Room temperature tensile and impact properties of the composition set forth in Table V are shown in Table VI as follows:

TABLE VI 0.2% Offset Yield Ultimate Tensile Elongation in 2 Chnrpy V-Notch Strength, p.s.i. Strength, p.s.i. in., Percent Impact Tough- As a further example of ductility, bend test data were also obtained with samples of the alloy composition set forth in Table V. Strips 3 inches wide were cut from a /1 inch plate of this composition that had been normalized for one hour at 1700 F. and air cooled to a hardness of Rockwell C 46.5 (BI-IN 444). These samples were bent in a three-point loading fixture to a U-configuration of three-inch radius without developing any cracks.

From the foregoing, it will be seen that I have devised an improved low-alloy, high tensile strength steel in which the strength thereof appears to be associated with an essentially all-martensitic structure produced by normalizing or by air-cooling following hot rolling.

The foregoing description has been for the purpose of illustration only and is not lirnting to the scope of the invention which is set forth in the claims.

What I claim is:

1. A low-alloy, high-strength steel having a martensitic microstructure, said steel consisting of from 0.20% to 0.30% carbon, 0.80% to 1.20% manganese, 3.25% to 4.00% nickel, 1.25% to 2.00% chromium, 0.25% to 0.50% molybdenum, 0.20% to 0.50% silicon, sulphur and phosphorous each up to 0.04% maximum, and remainder iron.

2. A low-alloy, high-strength steel having a martensitic microstructure, said steel consisting essentially of from 0.24% to 0.26% carbon, 0.90% to 1.00% manganese, 3.40% to 3.60% nickel, 1.45% to 1.75% chromium, 0.25% to 0.35% molybdenum, 0.25% to 0.40% silicon, sulfur and phosphorus each up to 0.04% maximum, and remainder iron.

3. Normalized low-alloy steel chracterized by a thoroughly martensitic microstructure and substantially uniform hardness throughout a section having dimensions within up to 8 inches or more, said steel consisting of from 0.20% to 0.3% carbon, 0.80% to 1.20% manganese, 3.25% to 4.00% nickel, 1.25% to 2.00% chromium, 0.25% to 0.50% molybdenum, 0.20% to 0.50% silicon, sulphur and phosphorus each up to 0.04% maximum, and remainder iron.

References Cited UNITED STATES PATENTS 925,659 6/1909 Schneider 128 1,660,790 2/1928 Herman 75-128 X 2,012,765 8/1935 Marthourey 75128 X 2,327,490 8/1943 Bagsar 75128 X 2,791,500 5/1957 Foley 148*36 3,165,402 1/1965 Finkl 75128 3,254,991 6/1966 Shimmin 75128 3,266,947 8/1966 Steiner 14836 X OTHER REFERENCES Tool Steels, Roberts et 2.1., Third ed., A.S.M., 1962, pages 230-231, 272, 423, 424, 425, and 436.

CHARLES N. LOVELL, Primary Examiner.

DAVID L. RECK, Examiner.

P. WEINSTEIN, Assistant Examiner. 

